1. Field of the Invention
This invention relates to a method of and an apparatus for seeking a desired track in an optical information recording and/or reproducing apparatus.
2. Related Background Art
Various types of media using light to record information thereon and read out the recorded information, there are known, such as a disc-like form, a card-like form and a tape-like form. These optical information recording mediums include ones capable of recording and reproduction and ones capable of only reproduction.
Recording of information on a medium capable of recording is accomplished by scanning information tracks by a light beam modulated in accordance with record information and reduced to a minute spot-like shape, and information is recorded as an optically detectable information bit row.
Also, reproduction of information from a recording medium is accomplished by scanning the information bit row of information tracks by a light beam spot of such predetermined power that recording is not effected on the medium, and detecting the light reflected from or transmitted through the medium.
In an apparatus for information recording on the recording medium as described above and information reproduction from the recording medium, a so-called optical head is used for the application of a light beam spot to the recording medium and the detection of light reflected from or transmitted through the medium. This optical head is movable relative to the recording medium in the direction of the information tracks thereof and in a direction across said direction, and the information track scanning of the light beam spot is accomplished by such relative movement. Also, in the optical head, a portion of the optical system, for example, an objective lens, is independently held for movement relative to the optical head body in the direction of the optic axis thereof (the focussing direction) and in a direction orthogonal to both the direction of the optic axis and the direction of the information tracks of the recording medium (the tracking direction). The holding of the objective lens is generally done through a resilient member, and the movement of the objective lens in said two directions is usually effected by an actuator utilizing an electromagnetic interaction.
Of the optical information recording mediums as described above, a card-like optical information recording medium (hereinafter referred to as the "optical card") has a great estimated demand as an information recording medium of relatively large capacity, which is compact and light in weight and convenient to carry.
FIG. 1 of the accompanying drawings is a schematic plan view of a postscript type optical card, and FIG. 2 of the accompanying drawings is a fragmentary enlarged view thereof.
Referring to FIG. 1, a number of parallel information tracks 4 extending in a direction indicated by arrows L and F are arranged on the information recording surface of the optical card 1. A home position 3, which provides a the reference position for access to the information tracks 4, is also provided on the information recording surface of the optical card 1. The information tracks 4 are arranged in the order of 4-1, 4-2, 4-3, . . . in succession from the home position 3.
As shown in FIG. 2, tracking tracks (e.g. 5-1, 5-2, 5-3) are provided adjacent to the information tracks 4 (e.g. 4-1, 4-2, 4-3). The tracking tracks are utilized as a guide for auto tracking (AT) in which a light beam spot is controlled so as not to deviate from a predetermined information track during the scanning of the light beam spot during information recording and reproduction.
As shown in FIG. 2, G marks 7-1, 7-2 and patterns for discrimination 8-1, 8-2 are pre-recorded on the information tracks 4-2 and 4-3, respectively. The G marks are indicative that these information tracks are fiducial tracks, and the patterns for discrimination are indicative of information regarding the optical card and the information tracks. A suitable number of such fiducial tracks may be provided. The G marks and the patterns for discrimination are formed by preformat, like the tracking tracks, or formed by recording using a light beam spot.
During the recording or reproduction of information, the light beam spot first lies at the home position 3, and is then accessed from this position to a desired information track in the direction of arrow D.
The preformat of such an optical card is described in detail in copending U.S. Patent application Ser. No. 07/036,557.
In order to cause a light beam spot to scan without deviating from a desired information track when the information track is scanned by the light beam spot during the recording or reproduction of information, auto tracking (AT) servo is executed. This AT servo is accomplished by detecting the deviation of the light beam spot from the information track (the AT error) in the optical head, negatively feeding back the detection signal to the tracking actuator, moving the objective lens relative to the optical head body in the tracking direction and causing the light beam spot to follow a desired information track.
Also, in order to make a light beam into a spot of proper size (i.e., to focus the light beam) on the optical card surface when the information track is scanned by the light beam spot during the recording or reproduction of information, auto focusing (AF) servo is executed. This AF servo is accomplished by detecting the deviation of the light beam spot from its focused state (the AF error) in the optical head, negatively feeding back the detection signal to the focusing actuator, moving the objective lens relative to the optical head body in the focusing direction and focusing the light beam spot onto the optical card surface.
The operation of accessing the light beam spot to a desired track is accomplished in the following manner.
First, the light beam spot S lies at the home position 3, where the AT servo draw-in is effected. Thereafter, the light beam spot is moved in the direction of arrow D by driving the optical head. Each time the light beam spot crosses a tracking track, a track crossing detection signal appears from a photodetector in the optical head. FIG. 3 of the accompanying drawings shows the track crossing detection signals. That is, when the light beam spot crosses the tracking tracks 5-1, 5-2, 5-3, 5 4, signals T1, T2, T3, T4 appear. At a point of time whereat a desired number has been counted by counting these signals, the AT servo draw-in is effected and servo draw-in to a desired fiducial track is effected. Further, when access is made from the desired fiducial track to a desired track proximate thereto, the kick operation of moving the objective lens relative to the optical head body in the tracking direction is performed.
Such a track searching method is disclosed, for example, in U.S. Pat. No. 4,106,058 or U.S. Pat. No. 4,285,015. Thus, in the prior-art track searching method as described above, when a detection signal similar to the detection of the tracking tracks appears due to a defect of the recording medium, such as dust adhering to the surface of the recording medium or a flaw present on the surface of the recording medium, such a signal is counted as a track crossing detection signal. This has led to the difficulty that the light beam spot accesses to a wrong track and the access to a desired fiducial track cannot be accomplished accurately.